1. Field of the Invention
This invention relates to a process and apparatus for the combustion of special wastes and the vitrification of fine dusts in a rotary tubular kiln. Combustion installations with rotary tubular kilns are primarily designed and constructed to burn solid, pasty, sludgy and viscous special wastes, i.e. extremely heterogeneous mixtures of waste materials which can be delivered continuously, but which are delivered primarily in batches, and frequently only in containers. Combustion of such wastes in conventional household waste combustion installations would be problematic, and therefor special combustion facilities are needed.
2. Background Information
German Laid Open Patent Appln. No. 28 08 637 discloses the combustion of special waste substances in installations with rotary tubular kilns in which the rotary tubular kiln empties directly into an afterburner. The molten slag produced in the rotary tubular kiln is transported to a wet slag removal device positioned at the end of the rotary tubular kiln, underneath the afterburner chamber.
From the afterburner chamber or from the kiln charging side, a lance can be used to blow fine dust in an air current into the molten slag bath of the rotary tubular kiln. This bonds the fine dusts containing heavy metals into the slag during the burning of the waste. An alternative proposal is that the fine dusts be pelletized by means of water and binders, and then transported in containers to the rotary tubular kiln, as is done with the waste substances. An auxiliary burner which is operated with waste oil can be used to assist the combustion process in the rotary tubular kiln or in a molten slag bath inside the afterburner chamber.
This method, however, has several disadvantages. First, the combustion can be conducted only superstoichiometrically, and second, to the extent that fine dusts are introduced by an air current, a great deal of polluted air is forced through the installation, and the thermal conditions in the rotary tubular kiln allow only a very limited integration of the dusts into the slag.
On account of the various waste materials, solid, pasty, sludgy and viscous, of which the composition and combustion behavior vary greatly, extremely heterogeneous waste gases are generated in the rotary tubular kiln, both with regard to the gas composition and the gas combustion temperature.
The requirements for such rotary tubular kilns are:
1. absolute burning of the remaining solid residues which is possible in practice only with molten slags; and PA0 2. the greatest possible burning of the waste gases, so that, in an afterburner chamber located behind the rotary tubular kiln, the burning limits for the allowable air pollution standards can be met. PA0 a) the amount of waste gas, the waste gas temperature and the resulting velocity of the waste gas, PA0 b) the hold time in the rotary tubular kiln, determined by the kiln inclination, the speed of rotation, angle of repose of the waste material, the melting behavior of the waste and the slag and the viscosity of the liquid slag, PA0 c) the reaction surface area, determined for example by the grain size of the waste, the density of the waste, the content of inorganic material, the waste melting behavior and the charging of the individual kiln zones, i.e. the drying, degasification, combustion and afterburning zones, PA0 d) and additional control variables, e.g. the number and size of the containers and waste charges delivered, the proportion of skin-forming substances in the slag, the concentration of salts and salt forming substances in the waste, and the possibility of adding the waste to the kiln in a uniformly-dosed manner. PA0 22 to 30% solid and pasty wastes to PA0 78 to 70% liquid waste atomized via burners. PA0 60 to 70% solid and paste wastes to PA0 40 to 30% liquid waste atomized via burners.
In the past, the simultaneous fulfillment of these two requirements has been possible only by using large amounts of fluid waste which has a high caloric value and which also can be sprayed into the furnace via burners.
The proportion of these wastes which can be atomized, however, should be kept as low as possible for economic reasons, since such wastes can be processed more economically or can be disposed of in installations with combustion chambers. Frequently, the ratio of liquid to solid waste is out of balance, so that without additional fuels such as heating oil or natural gas, it is impossible to simultaneously meet both these requirements.
In practice, the combustion air that is not delivered by means of burners, is generally kept constant. Many attempts have been made to control the combustion air as a function of the oxygen requirement for optimal combustion, but none have ever fulfilled expectations. In batch operation, and in particular for container operation, the energy content and the combustion behavior of the wastes cannot be sufficiently estimated because the parameters; energy content, proportion of inorganic material and water, pellet size, melting behavior, degasification, reaction surface, flammability and other characteristics, can seldom be adequately determined in advance.
Moreover, the current proportion of solid matter, and thus the amount of material actually contained in a batch, may vary on account of the changing composition.
Batch operation of a combustion facility produces peak loads, and the amount of oxygen in the combustion air must be set accordingly.
To achieve a sufficient burning of the waste gas in the rotary tubular kiln even with the above-mentioned peak loads, the following minimum combustion air excesses have proven effective in practice:
greater than 1.35 for liquid waste with delivery via burners;
greater than 2.00 for continuously delivered, sludgy and pasty wastes;
greater than 3.00 for waste delivered in batches as bulk material; and
greater than 3.00 for waste delivered in batches in containers.
The average combustion air excess is generally set at a value in the neighborhood at 2.5, to meet all the requirements.
Operation with viscous slags is possible at waste gas temperatures between 1050 and 1300 degrees C, with a tendency to 1300 degrees C, as a function of the composition of the inorganic waste components and possible additives. An optimal burning of the solid residues is possible only with molten slags.
For a theoretically average combustion air excess of 2.75, in relation to solid and semi-solid waste, it can be calculated that with a waste gas temperature of 1250 degrees C, only approximately 22% of the energy resulting from the waste, in relation to the lower combustion value Hu, can come from solid, sludgy and pasty waste, with the remainder having to come from liquid waste or supplementary fuel. This is not economical.
As a result of the extremely heterogeneous waste material and the high combustion air excesses, heterogeneous waste gases are formed and a correspondingly poor burning of the waste gas is achieved in the rotary tubular kiln.
In general, these rotary tubular kiln waste gases are transported directly into an afterburner chamber, where, if necessary, the temperature is then raised by the addition of liquid or gaseous fuels, and a remaining oxidation of the waste gases is conducted at low waste gas velocity and during a long hold time. This process and the construction of such furnaces with afterburners directly connected to the kilns, do not allow an intensive mixing of the waste gases. Because a good mixture of the waste gases is not achieved in the afterburner chamber and thus only a limited burning of the waste gases is occurring, the waste gases must be burned as much as possible in the rotary tubular kiln before they get to the afterburner. In relation to the above-mentioned conditions, such as combustion air excess, waste gas temperature, ratio of solid and liquid wastes, and with a typical diameter to length ratio of the rotary tubular kiln of 1:3.2, for example, approximately 100,000 to 150,000 Kcal/m.sup.3 h (approximately 420,000 to 640,000 kJ/m.sup.3 h) can be processed.
This value is a function of:
In the installations of the prior art, the control of the molten slag flow and thus the vitrification of the slag is even more difficult than controlling the amount of the combustion air oxygen.
On account of the extremely high combustion air excesses in the rotary tubular kiln, a great deal of polluted air is forced through the system and needs to be heated. The efficiency of the furnace is thereby drastically reduced. With primarily solid waste material with a low caloric value, therefore, a great deal of heating oil or natural gas must be added to maintain the required minimum temperatures.